Yep but i was kind of 10 years old at the time, no real experience. When i got them they were about 3 cm each. And after they ate the dollars it was only the two of them along with a cynodis (the third had died from fungus) which they didnt even dare to approach. Into that 10+ years they grew 15cm+ (dont know the inches sorry).
EDIT: BTW i am not sure how much a US gallon is. My tank is 160 liters. I say 36 due to a converter.